Acrylic esters can be represented by the general formula CH.sub.2 .dbd.CR.sup.1 COOR.sup.2, wherein R.sup.1 represents a hydrogen atom or an optionally substituted hydrocarbyl group, preferably a hydrogen atom or a methyl group and R.sup.2 represents an optionally substituted hydrocarbyl group, typically an alkyl group such as a methyl group. The invention relates in particular to refined methyl acrylate and, more particularly, methyl methacrylate (hereinafter MMA).
Acrylic esters are valuable compounds that readily polymerize or copolymerize with a wide variety of other monomers. The reverse side of the coin is that it is difficult to prepare acrylic esters of sufficient purity without inadvertently converting part of the acrylic esters into oligomers, polymers or other undesirable products during their preparation.
The various methods for preparing acrylic esters are discussed in detail in volume 1 of the fourth edition of the Kirk-Othmer Encyclopedia of Chemical Technology (pp. 287-314, in particular FIG. 1 on p. 296). The same reference provides a typical process for preparing acrylic esters by esterification (pp. 301-302, FIG. 4) wherein acrylic acid, alcohol and a catalyst are fed to an esterification reactor. The esterification products (acrylic ester, excess alcohol and water of esterification) are taken overhead from the distillation column. The process uses additional wash and distillation columns to yield a refined acrylic ester. To minimize formation of the acrylic ester oligomers, polymers, etc., mild conditions and short residence times are maintained throughout the separation sections. In addition, conventional free-radical polymerization inhibitors are fed to each of the distillation columns. Finally, monomethyl ether of hydroquinone (10-100 ppmw, hereinafter MEHQ) is added to the end product as polymerization inhibitor and the esters are used in this form in most industrial polymerizations. This reference also discusses alternative routes to acrylic esters, based on acetylene, etc., that each incorporate a final (series of) distillation column(s) to separate refined acrylic ester. Indeed, as the alternative routes give rise to a multitude of difficult to separate side-products, distillation to separate refined acrylic ester is essential.
The Process Economics Program Report No. 11D, entitled METH-ACRYLIC ACID AND ESTERS" by SRI International of January, 1993, provides a further thorough review of the routes to MMA and methacrylic acid. Concerning MMA purification, suggested treatments include distillation, extraction, and treatment over ion-exchange resins or carbon beds. To avoid polymerization during purification, MMA must not be exposed to high temperatures, and distillation is generally done under vacuum and in the presence of polymerization inhibitors (pp. 5-4 to 5-5). In section 9 of the Report (MMA from propyne) it is proposed to distil under vacuum conditions to keep temperatures low and minimize decomposition or polymerization. In the Report it is suggested to add about 600 ppmw of MEHQ as polymerization inhibitor (p.9-5).
U.S. Pat. No. 4,518,462 discloses a 2 column distillation process for removing water, methanol and methyl isobutyrate from MMA by using an n-hexane entrainer. In the examples, 0.02% of hydroquinone and 0.03% of phenothiazine are used as polymerization inhibitors in the first distillation column. In the second distillation column, 5% of hydroquinone is used.
In Japan Kokai 2-17151 (1990, abstract) distillation of impure MMA in the presence of 200 ppmw 4-benzoquinone plus 800 ppmw phenothiazine is described.
Neither of these documents suggest measures to improve the efficiency of phenolic inhibitors, or identify the inhibitors used best in case the acrylic ester containing stream further comprises (the residue of) the catalyst system used to prepare the acrylic ester.
The inventors set out to determine the most optimal conditions for distillation of the acrylic ester, allowing for high throughputs by distilling at elevated temperatures, without significant loss of the acrylic ester. In particular, it is an object of the present invention to provide a process that allows distillation of the product stream of a carbonylation reaction further comprising (i) (residue of) the homogeneous catalyst system used to prepare the acrylic ester and (ii) optionally an alcohol corresponding to the alcohol residue of the acrylic ester. More in particular, it is an object of the present invention to provide a process that allows the distillation of a product stream of a carbonylation reaction catalyzed by a catalyst system based on: (a) a Group VIII metal, preferably Pd; a ligand, preferably a phosphine mono- or bidentate ligand, and (c) a source of anions (Reppe Reactions, see chapter 3 of New Syntheses with Carbon Monoxide by J. Falbe, ISBN 3-540-09674-4). Such distillations are complicated by the presence of these catalyst components as they may induce formation of the undesired oligomers, polymers etc.